Pseudo-differential output driver with high immunity to noise and jitter

ABSTRACT

Circuits and methods are provided for transmitting a pseudo-differential output signal with relatively high immunity to noise and jitter. The output driver of the invention receives two differential input signals and outputs a single output signal with low voltage transistors and programmable impedance and on-die termination circuits. The pseudo-differential output driver consumes little circuit area and has low output capacitance.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/137,978, filed May 25, 2005, now U.S. Pat. No. 7,365,570, which ishereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to an integrated circuit driver. Moreparticularly, this invention relates to a pseudo-differential outputdriver.

Drivers are common circuits used for transmission of signals, eitherbetween integrated circuits (“ICs”) or within a single IC. For instance,driver circuitry can be used to transmit (1) data to and from a memoryor (2) control signals to and from a memory controller. There are manytypes of drivers, each with their own advantages and disadvantages.

One type is a “single-ended” output driver, which receives an inputsignal and generates an output signal. Information is typically conveyedby the voltage of the output signal, which is substantially equal toeither a relatively high voltage, such as that of a power supply(“VCC”), or a relatively low voltage, such as that of ground (“VSS”).Single-ended drivers are relatively simple, consume relatively littlearea, require a relatively low pin count, and have relatively lowcapacitance. However, they are quite susceptible to noise and jitter,such as that caused by unregulated power supply voltages.

In contrast, a “differential” output driver receives substantiallyconcurrently two input signals and generates substantially concurrentlytwo output signals. Information is typically conveyed by the voltagedifference between the two output signals, with one voltagesubstantially equal to VCC and the other voltage substantially equal toVSS. Such pairs of input and output signals are known as differentialsignals. Differential output drivers are relatively immune to noise andjitter, because any noise or jitter that affects the voltage of oneoutput signal will often affect in substantially the same way thevoltage of the other output signal. However, differential drivers areoften relatively complex, consume a relatively large amount of area,require a relatively high pin count, and have relatively highcapacitance.

In view of the foregoing, it would be desirable to be able to provide asingle-output driver with greater immunity to noise and jitter, whilemaintaining small size and low driver capacitance.

SUMMARY OF THE INVENTION

In accordance with this invention, a “pseudo-differential” output driverwith relatively high immunity to noise and jitter is provided. Theinvention preferably includes two transistor chains coupled in parallel,each chain receiving a respective signal of a pair of differentialsignals. Each chain preferably includes two complementary transistorscoupled in series. The pseudo-differential output is advantageouslycoupled to either one of the two chains, at a node that connects the twocomplementary transistors of that chain. An embodiment of the inventionpreferably includes programmable impedance circuitry coupled between VCCand the two transistor chains. In addition, programmable impedance andon-die termination (“ODT”) circuitry is preferably coupled between thetwo transistor chains and VSS. These programmable circuitries can beused to program signaling voltage levels or to adjust for transistor andresistor process variations.

In another embodiment of the invention, the pseudo-differential outputis preferably also coupled to VSS through the above programmableimpedance and ODT circuitry. The output is also preferably coupled toVCC through another programmable impedance and ODT circuitry. Theseprogrammable circuitries can further adjust the voltage swing of theoutput signal and can be used to set the ODT values when the driverperforms a read or termination transaction.

The invention advantageously generates a pseudo-differential outputsignal using two differential input signals. As a result, greaterimmunity to noise and jitter is achieved, as compared to a traditionalsingle-ended driver.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1 is a circuit diagram of an illustrative single-ended outputdriver;

FIG. 2 is a circuit diagram of an embodiment of a pseudo-differentialoutput driver in accordance with the invention;

FIG. 3 is a circuit diagram of an embodiment of a pull-up impedancecircuit in accordance with the invention;

FIG. 4 is a circuit diagram of an embodiment of a pull-up impedancesub-circuit in accordance with the invention; and

FIG. 5 is a block diagram of an illustrative system incorporating theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a known single-ended output driver 100. Single-endedoutput driver 100 includes driver blocks 102 coupled in parallel betweena source of relatively high voltage (“VCC”) and a source of relativelylow voltage (“VSS”). Each driver block 102 includes P-type metal-oxidesemiconductor (“PMOS”) transistor 104, resistors 106 and 108, and N-typemetal-oxide semiconductor (“NMOS”) transistor 110, all serially coupledbetween VCC and VSS. Although single-ended output driver 100 includesthree driver blocks 102, any suitable number of driver blocks 102 can beused.

Each driver block 102 accepts a single bit IN[N] of input signal bus IN,which is coupled to the gates of PMOS transistor 104 and NMOS transistor110, and drives output signal OUT, which is coupled between resistors106 and 108. Each driver block 102 behaves substantially like aninverter without a full rail-to-rail voltage swing. That is, when itsinput signal IN[N] represents a logical 0, PMOS transistor 104 isactivated and NMOS transistor 110 is deactivated, coupling output signalOUT to VCC through resistor 106 and driving output signal OUT to avoltage substantially equal to, but not necessarily exactly, alogical 1. On the other hand, when its input signal IN[N] represents alogical 1, NMOS transistor 110 is activated and PMOS transistor 106 isdeactivated, coupling output signal OUT to VSS through resistor 108 anddriving output signal OUT to a voltage substantially equal to, but notnecessarily exactly, a logical 0. By coupling a plurality of driverblocks 102 in parallel between VCC and VSS, the overall resistance ofsingle-ended output driver 100 can be varied by applying appropriatevalues to input signal bus IN.

Although single-ended output driver 100 is relatively simple, it suffersfrom many of the noise and jitter problems of traditional single-endedoutput drivers. Because only one input signal is used, the noise andjitter rejection of differential signals cannot be taken advantage of.

FIG. 2 is a circuit diagram of an embodiment of a pseudo-differentialoutput driver 200 in accordance with the invention. Pseudo-differentialoutput driver 200 preferably includes impedance circuit 202, PMOStransistor 204, NMOS transistor 206, PMOS transistor 210, NMOStransistor 212, impedance and on-die termination (“ODT”) circuit 208,impedance and ODT circuit 214, and resistors 216, 218, and 220.

Impedance circuit 202, switching circuitry 203 (which includestransistors 204, 206, 210, and 212), and impedance and ODT circuit 208are coupled in series between a source of relatively high voltage (VCC)and a source of relatively low voltage (VSS). In addition, node 217 ispreferably coupled to impedance and ODT circuit 214 through resistor216, to impedance and ODT circuit 208 through resistor 218, and topseudo-differential output node OUT through resistor 220.

Differential input signals IN and IN_ are received by switching circuit203. In particular, input signal IN is applied to the gates of PMOStransistor 204 and NMOS transistor 206, while input signal IN_ isapplied to the gates of PMOS transistor 210 and NMOS transistor 212.Pseudo-differential output node OUT is preferably coupled throughresistor 220 to the drains of PMOS transistor 210 and NMOS transistor212, which are coupled to each other.

Input signals IN and IN_ are complementary signals, i.e., one will besubstantially equal to a logical 1 while the other will be substantiallyequal to a logical 0. Thus, at any given time during the operation ofpseudo-differential output driver 200, only one PMOS transistor and oneNMOS transistor in switching circuit 203 is activated. Output signal OUTis preferably pulled up or down, towards VCC or VSS, depending onwhether IN represents a logical 1 and IN_ represents a logical 0, orvice versa. Because both differential input signals IN and IN_ are usedto drive output signal OUT, pseudo-differential output driver 200 iscapable of greater immunity to noise and jitter than single-ended outputdriver 100.

Impedance circuit 202 affects the voltage swing of output signal OUT byproviding programmable pull-up resistance. Impedance circuit 102preferably uses independent power source VDD, whose voltage typicallylies between VCC and VSS, as a reference. Relatively low voltage VDD isoften used by the control logic of an IC, resulting in lower powerconsumption and small transistor sizes, while relatively high voltageVCC is reserved for more power-intensive circuits. Signal PU_ADJUST setsthe resistance of impedance circuit 202, which can also be used toaccount for process variation in transistors and resistors ofpseudo-differential output driver 200. Signal IMP_ENABLE enables ordisables impedance circuit 202.

Impedance and ODT circuit 208 affects the voltage swing of output signalOUT by providing programmable pull-down resistance. Impedance and ODTcircuit 208 preferably uses VDD as a reference. The resistance ofimpedance and ODT circuit 208 can be set by signal PD_ADJUST, which canalso be used to adjust for process variation in transistors andresistors of pseudo-differential output driver 200. Circuit 208 can beenabled or disabled by signal IMP_ENABLE, which is also used byimpedance circuit 202. On-die termination values of impedance and ODTcircuit 208 can be adjusted by signal ODT_ADJUST, and the ODT of circuit208 can be enabled or disabled by signal ODT_ENABLE. For instance, inthe context of a memory interface, the ODT of impedance and ODT circuit208 can be disabled during a write transaction and enabled during a readtransaction.

Similarly, impedance and ODT circuit 214 affects the voltage swing ofoutput signal OUT by providing programmable pull-up resistance.Impedance and ODT circuit 214 preferably uses VDD as a reference. Theresistance of impedance and ODT circuit 214 can be set by signalPU_ADJUST_2, which can also be used to account for process variation intransistors and resistors of pseudo-differential output driver 200. TheODT of circuit 214 can be enabled or disabled by signal ODT_ENABLE,which is also used by impedance and ODT circuit 208. For instance, inthe context of a memory interface, the ODT of impedance and ODT circuit214 can be disabled during a write transaction and enabled during a readtransaction.

FIG. 3 is a circuit diagram of an embodiment of a pull-up impedancecircuit 300 in accordance with the invention. Pull-up impedance circuit300 can be used as impedance circuit 202 of FIG. 2. Note thatimplementations of impedance circuit 202, impedance and ODT circuit 208,and impedance and ODT circuit 214 should be known to those of ordinaryskill in the art. Thus, pull-up impedance circuit 300 is shown simplyfor illustrative purposes.

Pull-up impedance circuit 300 preferably includes a plurality of logicalNOR gates 302 and a plurality of pull-up impedance sub-circuits 304. NORgates 302 preferably use voltage VDD, which is preferably lower thanpower supply voltage VCC. Although four logical NOR gates 302 and fourpull-up impedance sub-circuits 304 are shown, other numbers could beused. Each logical NOR gate 302 preferably receives signal PU_ENABLE andan input bit from control signal PU_ADJUST, and preferably generates anoutput signal PU_CTRL to a corresponding pull-up impedance sub-circuit304. Each pull-up impedance sub-circuit 304 preferably outputs signalPU_BIAS, which preferably couples to switching circuitry 203 of FIG. 2.

Pull-up impedance circuit 300 can be disabled by applying a logical 1 tosignal PU_ENABLE, which sets PU_CTRL to a logical 0 and deactivatespull-up impedance sub-circuits 304. The impedance of pull-up impedancecircuit 300 can be adjusted by applying a logical 0 to signal PU_ENABLEand applying appropriate voltages to the bits of signal PU_ADJUST. Inthis way, a specified subset of pull-up impedance sub-circuits 304 canbe activated, thereby setting the impedance of pull-up impedance circuit300. In a preferred embodiment of pull-up impedance circuit 300, eachpull-up impedance sub-circuit 304 carries a different amount ofresistance.

FIG. 4 is a circuit diagram of an embodiment of a pull-up impedancesub-circuit 400 in accordance with the invention. Pull-up impedancesub-circuit 400 can be used as pull-up impedance sub-circuit 304 of FIG.3, and preferably includes NMOS transistor 402 and resistor 404. NMOStransistor 402 preferably serves as an enable device, responsive tosignal PU_CTRL, which is applied to the gate of NMOS transistor 402.

The concepts and principles illustrated in FIGS. 3 and 4 can be extendedto implementations of impedance and ODT circuits 208 and 214, withappropriate adjustments. For instance, a pull-down impedance sub-circuitin impedance and ODT circuit 208 preferably uses an NMOS transistor asan enable device, and couples this transistor between the correspondingvariable resistor and voltage source VSS.

Because of the differential inputs, pseudo-differential output drivercircuit 200 advantageously provides greater immunity to noise and jitterthan traditional single-ended drivers. An embodiment of the inventionpreferably supports programmable resistance and on-die termination, toset parameters such as the voltage swing of the pseudo-differentialoutput signal. In addition, the stacking of transistors in impedancecircuit 202, switching circuitry 203, and impedance and ODT circuit 208preferably allows lower-voltage transistors to be used, since thegate-to-source breakdown voltages of the transistors will typically notbe exceeded. Lower-voltage transistors typically have significantlyshorter gates, thereby resulting in less area consumption. Also, the useof smaller transistors can significantly reduce the capacitance seen byoutput signal OUT. For instance, transistor sizes can be reduced byapproximately 30%, leading to a roughly 30% reduction in outputcapacitance.

Note that the embodiments of the invention described herein are forpurposes of illustration only, and other embodiments that do not deviatefrom the spirit and scope of the invention are possible. For instance,as discussed above with respect to FIG. 3, other numbers of NOR gates302 and pull-up impedance sub-circuits 304 can be used. In addition, NORgates 302 can be replaced by any logic gate suitable to the applicationat hand. As another example, the choice of NMOS transistors and PMOStransistors could be reversed. As yet another example, the MOStransistors could be replaced with bipolar junction transistors withoutdeviating from the spirit and scope of the present invention.

FIG. 5 shows a system that incorporates the invention. System 500includes a plurality of dynamic random access memory (“DRAM”) devices502, a processor 504, a memory controller 506, input devices 508, outputdevices 510, and optional storage devices 512. DRAM devices 502 includeat least one pseudo-differential output driver (e.g., driver 200) inaccordance with the invention. This driver transmits data or controlsignals with greater immunity to noise and jitter than traditionalsingle-ended output drivers. Data and control signals are transferredbetween processor 504 and memory controller 506 via bus 514. Similarly,data and control signals are transferred between memory controller 506and DRAM devices 502 via bus 516. Input devices 508 can include, forexample, a keyboard, a mouse, a touch-pad display screen, or any otherappropriate device that allows a user to enter information into system500. Output devices 510 can include, for example, a video display unit,a printer, or any other appropriate device capable of providing outputdata to a user. Note that input devices 508 and output devices 510 canalternatively be a single input/output device. Storage devices 512 caninclude, for example, one or more disk or tape drives.

Thus it is seen that circuits and methods are provided for transmittinga pseudo-differential output signal with relatively high immunity tonoise and jitter. One skilled in the art will appreciate that theinvention can be practiced by other than the described embodiments,which are presented for purposes of illustration and not of limitation,and the present invention is limited only by the claims which follow.

1. A circuit for transmitting an output signal, said circuit comprising:a first impedance operative to receive a first voltage, wherein saidfirst impedance comprises first resistor circuitry whose resistance isadjustable responsive to a voltage of a first resistance control signal;switching circuitry coupled to said first impedance, said switchingcircuitry comprising a first path and a second parallel path, saidswitching circuitry operative to receive first and second differentialinput signals at said first and second paths, respectively, and togenerate said output signal at a node in only one of said paths, withoutgenerating another output signal in another of said paths, in responseto receipt of said first and second differential input signals; and asecond impedance coupled to said switching circuitry and operative toreceive a second voltage, wherein said second impedance comprises secondresistor circuitry whose resistance is adjustable responsive to avoltage of a second resistance control signal.
 2. The circuit of claim 1further comprising: a third impedance coupled to said output node andoperative to receive said first voltage.
 3. The circuit of claim 2wherein said third impedance comprises third resistor circuitry whoseresistance is adjustable responsive to a voltage of a third resistancecontrol signal.
 4. The circuit of claim 2 wherein: said third impedanceis coupled to said output signal through fourth resistor circuitry; andsaid output signal is coupled to said second impedance through fifthresistor circuitry.
 5. The circuit of claim 1 wherein: said first pathcomprises first and second transistors coupled in series; and saidsecond path comprises third and fourth transistors coupled in series. 6.The circuit of claim 5 wherein: gates of said first and secondtransistors are coupled to said first differential input signal; andgates of said third and fourth transistors are coupled to said seconddifferential input signal.
 7. The circuit of claim 6 wherein: said firstand second transistors are complementary; and said third and fourthtransistors are complementary.
 8. The circuit of claim 6 wherein: saidoutput signal is coupled to a connection between said third and fourthtransistors.
 9. The circuit of claim 1 wherein: said first impedancefurther comprises a first transistor coupled in series to said firstresistor circuitry; and said second impedance further comprises a secondtransistor coupled in series to said second resistor circuitry.
 10. Amethod for transmitting an output signal from a driver circuit, saidmethod comprising: receiving a first differential input signal at afirst path; receiving a second differential input signal at a secondparallel path, said second differential input signal complementary tosaid first differential input signal; driving said output signal at anode in only one of said paths, without driving another output signal inanother of said paths, a voltage of said driven output signal beingindicative of a difference in voltages of said first and seconddifferential input signals; and disabling first on-die terminationcircuitry coupled in series to said first and second parallel paths byapplying a first enable/disable signal to said first on-die terminationcircuitry.
 11. The method of claim 10 further comprising: activating afirst transistor and deactivating a second transistor responsive to avoltage of said first differential input signal, wherein said first andsecond transistors are coupled in series; and activating a thirdtransistor and deactivating a fourth transistor responsive to a voltageof said second differential input signal, wherein said third and fourthtransistors are coupled in series.
 12. The method of claim 11 whereinsaid activating said first transistor and said deactivating said secondtransistor couple said output signal to a source of substantiallyconstant voltage through programmable impedance circuitry.
 13. Themethod of claim 10 further comprising: setting a resistance of firstimpedance circuitry coupled in series to said first and second parallelpaths by applying a first control signal to said first impedancecircuitry; and setting a resistance of second impedance circuitrycoupled in series to said first and second parallel paths by applying asecond control signal to said second impedance circuitry.
 14. The methodof claim 13 further comprising: enabling said first impedance circuitryby applying a first enable/disable signal to a transistor of said firstimpedance circuitry; and enabling said second impedance circuitry byapplying a second enable/disable signal to a transistor of said secondimpedance circuitry.
 15. The method of claim 10 further comprisingdisabling second on-die termination circuitry coupled to said outputsignal by applying a second enable/disable signal to said second on-dietermination impedance circuitry.
 16. A system comprising: a processor; amemory controller; a dynamic random access memory device comprising anarray of memory cells and a circuit for transmitting an output signal,said circuit comprising: a source of relatively low voltage; a firstimpedance operative to receive a first voltage, wherein said firstimpedance comprises first resistor circuitry whose resistance isadjustable responsive to a voltage of a first resistance control signal;switching circuitry coupled to said first impedance, said switchingcircuitry comprising a first path and a second parallel path, saidswitching circuitry operative to receive first and second differentialinput signals at said first and second paths, respectively, and togenerate said output signal at a node in only one of said paths, withoutgenerating another output signal in another of said paths, in responseto receipt of said first and second differential input signals; and asecond impedance coupled to said switching circuitry and operative toreceive a second voltage, wherein said second impedance comprises secondresistor circuitry whose resistance is adjustable responsive to avoltage of a second resistance control signal.
 17. The system of claim16 wherein: said first path comprises first and second transistorscoupled in series; and said second path comprises third and fourthtransistors coupled in series.
 18. The system of claim 17 wherein: saidfirst and second transistors are coupled to said first differentialinput signal; and said third and fourth transistors are coupled to saidsecond differential input signal.
 19. The system of claim 18 whereinsaid output signal is coupled to a connection between said third andfourth transistors.
 20. A circuit for transmitting an output signal froma driver circuit, said circuit comprising: means for receiving a firstdifferential input signal at a first path; means for receiving a seconddifferential input signal at a second parallel path, said seconddifferential signal complementary to said first differential signal;means for driving said output signal at a node in only one of saidpaths, without driving another output signal in another of said paths, avoltage of said driven output signal being indicative of a difference involtages of said first and second differential input signals; and meansfor disabling first on-die termination circuitry coupled in series tosaid first and second parallel paths by applying a first enable/disablesignal to said first on-die termination circuitry.